We looked for evidence that the diffusion of heavy meromyosin is modified by its interaction with actin. To be able to observe diffusion in one dimension, we electrophoresed the complex of F-actin and heavy meromyosin in agarose gels in thin capillaries. The intensity profile of the electrophoretic band of the complex showed a sharp peak, which in 1% agarose in the electric field of 17.8 V cm-1 at room temperature migrated at 3.2 cm h-1. The time evolution of the profile after the electrophoresis ended was a measure of the diffusion of heavy meromyosin. After 10 min the intensity profile of heavy meromyosin diffusing in the presence of F-actin and ATP had undergone as much change as the profile of free heavy meromyosin. Modelling of the diffusion process showed that the mean diffusion coefficient of heavy meromyosin moving over actin in the presence of ATP was 7.2×10-7 cm2 s-1 and that it was not statistically different from the diffusion coefficient of free heavy meromyosin. This data is interpreted to show that the diffusion of heavy meromyosin is not modified by its interaction with actin.